Positive radiation sensitive resist terpolymers

ABSTRACT

This invention relates to novel terpolymers of 3-methylcyclopentene, an omega alkynoic acid and sulfur dioxide. Positive radiation sensitive films prepared from the subject terpolymers adhere well to the substrate, demonstrate resistance to cracking and erosion during development and possess excellent edge definition.

This invention relates to novel sulfone terpolymers which are useful aspositive radiation sensitive resist recording media for the manufactureof electronic devices including microelectronic circuits.

BACKGROUND OF THE INVENTION

Positive acting resist media sensitive to radiation, e.g. electron beam,and the use thereof in forming surface patterns in a variety ofsubstrates are well known in the art. Poly(olefin sulfone) polymers area major class of such positive resist media. For example,poly(1-methyl-1-cyclopentene sulfone) has been utilized for audio/videorecording apparatus as described in Poliniak et al. U.S. Pat. Nos.3,935,331 and 3,935,332. Poly(1-butene sulfone) has been employed forintegrated circuit mask formation and poly(cyclopentene sulfone) andpoly(bicycloheptene sulfone) have been employed for multilayerintegrated circuit mask formation.

In general, surface relief patterns are formed in a layer of apoly(olefin sulfone) or poly(acetylene sulfone) copolymer by dissolvingthe polymer in a suitable solvent, forming a wet layer of the resultingsolution on a substrate such as a metal, baking or vacuum drying the wetlayer to remove the solvent, irradiating the dry layer with e.g. amodulated beam of electrons, and developing the copolymer layer with asolvent to yield the desired surface relief pattern. To be useful as apositive resist media, such a film must degrade upon irradiation, bedevelopable in the exposed areas with good resolution, and be resistantto chemical etching solutions. Non-irradiated areas of the film mustadhere well to the substrate and be resistant to cracking and crazing.

The choice of a specific polymer for use as a radiation sensitive resistmedia heretofore has necessitated a balancing of such properties. Forexample, in order to prepare a resist film having a high degree ofsensitivity, one generally will select a polymeric material having highmolecular weight and low molecular weight distribution. Theseparameters, however, usually contribute to polymer brittleness. Theconverse is often true with low molecular weight polymers. Other equallyimportant criteria, such as retention of resist sensitivity and fineline resolution, must be balanced as well in selecting a polymericmaterial.

For example, a positive resist coating of poly(3-methylcyclopentenesulfone), such as is described in E. S. Poliniak et al., U.S. Pat. No.4,153,741, when spun on a chromium substrate to a thickness greater thanabout 3000 angstroms may show varying degrees of cracking or crazingupon development, depending on the type and quality of the chromesubstrate, the coating and baking procedures, and the characteristics ofthe resist material itself. The addition of a conventional plasticizerwill eliminate cracking in such coatings up to a thickness of about 6000angstroms. However, there is a concomitant deterioration in fine lineresolution and resist sensitivity.

Poly(olefin sulfone) terpolymers and their use as radiation sensitiveresist recording media are likewise known in the art. For example,Gipstein et al. in U.S. Pat. No. 3,898,350 disclose terpolymers formedfrom an alpha olefin, sulfur dioxide, and a compound selected from thegroup consisting of cyclopentene, bicycloheptene and methylmethacrylate.

Gipstein et al. indicate that poly(olefin sulfone) terpolymers such asare disclosed therein have lower glass transition temperatures and are,therefore, less brittle than copolymers made from the disclosed olefins,e.g. cyclopentene and bicycloheptene. The disclosed terpolymers areindicated as being resistant to cracking in films up to 9100 angstromsthick. In contrast, sulfur dioxide copolymers of the disclosed group ofmonomers represented by cyclopentene and bicycloheptene exhibit crackingin film thicknesses greater than about 3000 angstroms. The terpolymersdisclosed by Gipstein et al. are of little practical value, however,since their sensitivities appear to be above the 1 μC/cm² dose levelgenerally recognized as an upper limit for a commercial high throughputradiation sensitive resist.

The present invention is predicated on the discovery that small amountsof an omega alkynoic acid as a termonomer in a sulfone terpolymersignificantly improve certain desirable characteristics of radiationsensitive resist media formed therefrom without loss in sensitivity.

SUMMARY OF THE INVENTION

In accordance with this invention, there are provided novel sulfoneterpolymers suitable for the preparation of positive radiation sensitiverecording media which are characterized by improved adhesion to thesubstrate, reduced cracking and crazing, significantly reduced erosionof non-irradiated areas during development and improved edge definitionwithout loss in sensitivity in comparison to known poly(olefin sulfone)polymers conventionally utilized for such films.

DETAILED DESCRIPTION OF THE INVENTION

The novel sulfone terpolymers provided in accordance with the presentinvention are terpolymers of 3-methylcyclopentene, an omega alkynoicacid and sulfur dioxide. In terms of mole percentage, the terpolymers ofthis invention contain an organic monomer component i.e.,3-methylcyclopentene and an omega alkynoic acid, and sulfur dioxide in a1:1 molar ratio.

The organic monomer component comprises from about 93 mole percent toabout 99 mole percent of 3-methylcyclopentene and from about 1 molepercent to about 7 mole percent of an omega alkynoic acid. It has beenfound, in accordance with this invention, that reducing the content ofomega alkynoic acid of the subject terpolymers substantially below about1 mole percent of the monomer component will negate the improvedresistance to cracking which characterizes the subject terpolymers.Further, increasing the content of omega alkynoic acid substantiallyabove about 7 mole percent of the monomer component will not produce acorresponding increase in adhesion and resistance to cracking. As apractical matter, it is difficult to prepare the subject terpolymerswith an acid content in excess of about 10 mole percent of the monomercomponent because of the differences in the relative reactivities of themonomers. Therefore, it is preferred, in accordance with this invention,that the monomer component of the subject terpolymers comprise fromabout 2 mole percent to about 3 mole percent of omega alkynoic acid andthe remainder 3-methylcyclopentene.

The omega alkynoic acid termonomer of the terpolymers of this inventionis selected from those represented by the formula

    CH.tbd.C--(CH.sub.2).sub.n --COOH

wherein n is an integer from 1 to 20. A preferred termonomer is selectedfrom those of the above formula wherein n is an integer from 6 to 12,with 10-undecynoic acid, i.e. n equals 8, being especially preferred.The significant improvement in resist properties achieved by theinclusion of the omega alkynoic acid termonomers in the subjectterpolymers is surprising, particularly in view of the fact that similarterpolymers prepared using corresponding alkenoic acids are clearlyinferior in resist properties. For example, terpolymers of3-methylcyclopentene, sulfur dioxide and allylacetic acid or10-undecenoic acid, upon development after irradiation by electron beam,showed substantial erosion in non-irradiated areas leading tosignificant loss of edge definition. In contrast, the terpolymers ofthis invention show virtually no erosion in non-irradiated areas andhave excellent edge definition.

The molecular weight of the terpolymers of this invention should besufficiently high so that, when areas of films formed therefrom areirradiated by exposure to a radiation source, e.g. a modulated electronbeam, there will be a sufficient reduction in molecular weight to causea significant difference in the relative solubility of the irradiatedand non-irradiated portions of the film. The molecular weight of thesubject terpolymers is further important in terms of their solubility inthe solvents used for casting such films. In general, the novelterpolymers of this invention preferably have a weight average molecularweight (M_(w)) of from about 400,000 to about 1,500,000, more preferablyfrom about 700,000 to about 1,200,000, and a molecular weightdistribution (M_(w) /M_(n)) of less than 4, wherein M_(n) equals numberaverage molecular weight.

The novel sulfone terpolymers of this invention can be prepared in aconventional manner by a free radical polymerization of the monomercomponent and sulfur dioxide in the presence of a conventionalpolymerization initiator such as, for example, t-butylhydroperoxide. Themolecular weight of the terpolymer is controlled by the amount ofpolymerization initiator present with increased amounts giving lowermolecular weights.

In order to apply the terpolymers of this invention to a substrate, theterpolymer is initially dissolved in a suitable organic solvent. Theresulting solution is then coated onto the substrate in a conventionalmanner, e.g., by casting, spraying, spin coating and the like. Thesolution usually contains from about 1 percent by weight to about 20percent by weight, preferably from about 6 percent by weight to about 10percent by weight of the terpolymer. Suitable solvents should haveboiling points below the decomposition point of the terpolymer, thuspermitting removal of the solvent from the coating by conventionalheating or vacuum drying. Removal of the solvent forms a uniform resistfilm of the terpolymer on the substrate. Examples of suitable solventsfor the novel terpolymers of this invention include2-methoxyethylacetate and cyclopentanone.

The films are formed on the substrate in various thicknesses from about50 angstroms to about 10 microns, depending on the intended use of theresist image. It is preferred to bake the resist film in air or vacuum,usually at a temperature above the glass transition temperature of theterpolymer, but below its thermal decomposition point. It has been foundthat the resist performance of films of the terpolymers of thisinvention can be further improved in most instances by subsequentlyallowing the films to age for from one to several days, preferably atambient temperature.

The recording medium resist films of the terpolymers of this inventionare selectively irradiated with a source of radiation, e.g. a modulatedelectron beam, X-ray, gamma ray or the like, thereby causing degradationof the terpolymer in the irradiated areas. It is preferred, inaccordance with the present invention, to irradiate the subjectterpolymers with a modulated electron beam.

When utilized as positive-working resists, films of the terpolymers ofthis invention are cast to a thickness equal to or less than the depthof penetration of the radiation, thus uncovering the substrateunderlying the irradiated areas upon development. Alternatively, theterpolymers of this invention can be employed as media for recording ofsurface relief patterns by irradiating thicker films to form welldefined patterns which, upon development, correspond in relief to theinformation recorded.

Resist films formed from the novel terpolymers of this invention aredeveloped after irradiation utilizing conventional solvents. A solventmay be utilized which develops both the irradiated and non-irradiatedportions of the film but will dissolve the irradiated portions morerapidly. In this instance, resist thickness must be adjusted so that, inthe non-irradiated areas, sufficient resist remains after development toprotect the substrate during subsequent operations, e.g., etching. Theresist film may also be developed with a solvent which exclusivelydissolves the irradiated portion thus exposing the underlying substrate.This mode of development generally requires longer and more carefuldeveloping, but tends to give sharper resolution.

Suitable solvent developers for the terpolymers of this inventioninclude, for example, 2-methoxyethylacetate, 2-methylcyclohexanone,ethylacetoacetate and the like. Since these developers will dissolveboth the irradiated and non-irradiated portions of the resist, it ispreferred to utilize as a solvent system a combination of such a solventand a compatible non-solvent for the resist film, usually an alcoholsuch as, for example, isopropanol, ethoxyethanol, 2-methylcyclohexanoland the like. The formulation of such solvent/non-solvent combinationswhich will fully develop irradiated resist film while eroding only aninsignificant portion of the non-irradiated resist is conventional inthe art. In such systems, the effect of the non-solvent is to retard thedeveloping action of the solvent thereby affording more carefullycontrolled development.

The resist film, after removal of the irradiated portion, may provide apattern on the substrate which is used as a guide for the formation ofcircuits and the like. The substrates which may be advantageously sotreated include, for example, conductive plastic or a conductive plasticlaminate, porcelain-coated steel, silicon wafers, chrome- ornickel-coated glass, and the like, on which it is desired to form amicroelectronic circuit. In particular, resist films formed from theterpolymers of this invention are useful for the production of chromemasks which are in turn useful in the production of electronic devices.Exposed portions of the surface of the substrate, defined by theterpolymer resist, are etched by conventional etchant, e.g. ferricchloride solutions for steel, hydrofluoric acid solutions for glass andthe like.

The following examples further illustrate this invention, it beingunderstood that the invention is in no way intended to be limited to thedetails described therein. In the examples, all parts and percentagesare on a weight basis unless otherwise stated.

EXAMPLE 1

A solution of 8 gm 3-methylcyclopentene, 0.4 gm 10-undecynoic acid and0.1 ml of t-butylhydroperoxide as a polymerization initiator was addeddropwise to 30 ml of liquid sulfur dioxide with stirring at -30° C.under an inert atmosphere. After addition of the solution was completed,the excess sulfur dioxide was allowed to boil off. The residue wasdissolved in 100 ml tetrahydrofuran and the resulting solution slowlypoured into 250 ml methanol with agitation to precipitate the terpolymerproduct. The product was collected by filtration, reprecipitated fromtetrahydrofuran with methanol and dried overnight in a vacuum oven.Comparative quantitative infrared analysis of the product showed it tobe comprised of 3-methylcyclopentene/10-undecynoic acid/sulfur dioxidein a molar ratio of 97.5:2.5:100, yield greater than 70 percent.Molecular weight analysis by gel permeation chromatography showed M_(w)=1,219,571, M_(n) =412,431 and M_(w) /M_(n) =3.0.

EXAMPLE 2

For comparative purposes, a copolymer was prepared by dissolving 10grams of 3-methylcyclopentene and 0.25 ml t-butylhydroperoxide in 10 mlmethylene chloride to form a solution which was added dropwise to asolution of 15 ml anhydrous ethanol in 30 ml liquid sulfur dioxide at-30° C. under an inert atmosphere. After completion of the addition, theexcess sulfur dioxide was allowed to boil off and the precipitatedproduct recovered and purified in accordance with the procedure ofExample 1.

EXAMPLE 3

In order to demonstrate the enhanced adhesion of the terpolymers of thisinvention to a substrate, an undercut test was performed as follows. An8 percent by weight solution of the terpolymer of Example 1 in2-methoxyethylacetate was coated to approximately 5000 angstroms on achromium/glass substrate. The coating was baked for 30 minutes at 95° C.and then allowed to stand at ambient temperature for 24 hours. Thecoating, in each instance, was scored to the chromium and treated with aconventional cerric ammonium nitrate/glacial acetic acid chromiumetching solution for five minutes. The degree of undercut, which isrecognized as a definitive indicator of resist adhesion, was measuredvisually utilizing an optical microscope. The degree of undercut forthis resist was less than 0.005 mm, which is the limit of observation.In contrast, a film of the 3-methylcyclopentene/sulfur dioxide copolymerof Example 2 treated in a like manner showed an undercut of 0.025 mm.The degree of adhesion is particularly important with regard to chromiumetching since poor adhesion causes the developed film to lift off thesubstrate resulting in a loss of fine lines and edge definition.

EXAMPLE 4

In order to demonstrate the improvement in resistance to cracking of theterpolymers of this invention, a sample of the terpolymer of Example 1was treated according to the following procedure.

The terpolymer sample was dissolved in 2-methoxyethylacetate and spun tovarious thicknesses on a chromium/glass substrate, baked at 95° C. for30 minutes, and allowed to stand at ambient temperature for 24 hours.The films were placed in a developer consisting of an equal mixture of2-methylcyclohexanone and 2-methylcyclohexanol for from 70-130 seconds,depending on their thickness. The samples were then dipped in anisopropanol stopper, air dried, and observed under a microscope. In mostinstances, no cracking or significantly reduced cracking was observedwith the present terpolymers in comparison to the copolymer of Example2. The copolymer demonstrated cracking at thicknesses in excess of 5000angstroms whereas the terpolymer containing as little as 2.5 molepercent 10-undecynoic acid did not demonstrate cracking below about 8800angstroms. The terpolymer had the same sensitivity (0.8 μC/cm²) as thecorresponding copolymer.

EXAMPLE 5

Films of the terpolymer of Example 1 were cast on chromium/glasssubstrates to a thickness of 5000 angstroms in the manner of Example 3.Films of the copolymer of Example 2 were prepared in a similar manner.The films were irradiated by electron beam radiation through a maskconsisting of 3 mm bars and spaces at a dose of 0.8 μC/cm². Theterpolymer film was developed with a 55:45 mixture of2-methylcyclohexanone and 2-methylcyclohexanol. The terpolymer filmshowed excellent pattern definition and less than one percent erosion inthe non-irradiated areas. In contrast, the copolymer film, which wasdeveloped with a 60:40 mixture of the same solvents, was judged clearlyinferior in pattern definition and had approximately 4 percent erosionin the non-irradiated areas.

EXAMPLES 6-10

Terpolymers were prepared having varying ratios of 3-methylcyclopenteneand 10-undecynoic acid. In each instance, the organic monomer componentand 0.25 ml t-butylhydroperoxide were dissolved in 10 ml methylenechloride and the resulting solution added dropwise to a solution of 15ml anhydrous ethanol in 30 ml liquid sulfur dioxide with agitation at-30° C. under an inert atmosphere. After completion of the addition, theexcess sulfur dioxide was allowed to boil off. Each product wasrecovered and purified in accordance with the procedure of Example 1.The composition of each product was determined by comparativequantitative infrared spectroscopy.

EXAMPLE 11

A solution of 4.0 g 3-methylcyclopentene and 0.5 ml t-butylhydroperoxidewas slowly added to a stirred solution of 6.0 g 10-undecynoic acid in 30ml liquid sulfur dioxide in a reaction vessel at -10° C. under an inertatmosphere. After the addition was completed, the excess sulfur dioxidewas allowed to boil off.

The residue was dissolved in 100 ml tetrahydrofuran and the solutionpoured slowly into 250 ml of anhydrous diethyl ether with stirring toprecipitate the terpolymer product. The product was reprecipitated inthe same manner, dried overnight in a vacuum oven and analyzed bycomparative quantitative infrared spectroscopy.

Each of the terpolymers prepared in Examples 6 through 11 was testedaccording to the procedure of Example 4. In each instance, the minimumfilm thickness which showed evidence of cracking was determined. Theresults are given in Table I.

                  TABLE I                                                         ______________________________________                                                               Mole                                                                          Percent   Minimum                                      Terpolymer                                                                             Reactants(in Gms)                                                                           Acid** in Cracking                                     of Example                                                                             3-MCP*    Acid**  Product Thickness (μm)                          ______________________________________                                        6        9         0.25    ≃1.0***                                                                 0.62                                       7        8         0.4     2.5     0.88                                       8        7.5       0.8     3.4     0.96                                       9        10        3       3.3     1.00                                       10       4         4       4.0     0.96                                       11       4         6       9.6     1.07                                       ______________________________________                                         *3-methylcyclopentene                                                         **10-undecynoic acid                                                          ***One mole percent acid represents the limit of detection of acid conten     of the terpolymer.                                                       

The data in Table I shows reduced cracking up to a film thickness ofabout 1 μm. Minimum cracking thickness does not appreciably increasebeyond 1 μm despite further increases in the acid content of theterpolymer. In contrast, a film of the copolymer of Example 2 treated inthe same manner showed a minimum cracking thickness of 0.56 μm.

EXAMPLE 12

Eight percent by weight solutions of the terpolymer of Example 1 and thecopolymer of Example 2, respectively, in 2-methoxyethylacetate were caston chromium/coated glass to a thickness of 0.5 μm. The films were driedat 90° C. for 30 minutes and allowed to age overnight at roomtemperature. The films were irradiated with a computer controlledscanning electron beam in a pattern consisting of 3,1 and 0.5 μm linesat a dose of 0.8 μC/cm². After irradiation, the films were developed forone and one-half minutes with mixtures of 2-methylcyclohexane and2-methylcyclohexanol in ratios of 55:45 and 60:40, respectively.Development was stopped by immersing the film in an alcohol stopper. Thefilms were dried with compressed air, dipped in a conventional cerricammonium nitrate/glacial acetic acid chromium etching solution for threeminutes, rinsed with water and dried. Under a scanning electronmicroscope, the terpolymer film showed good edge definition of lineswith resolution better than 1 μm. The copolymer film, in contrast,showed undercutting and poor edge definition which is characteristic ofadhesion failure. One micron lines were missing and areas or "islands"of non-irradiated resist film tended to lift off of the substrateallowing the chromium etch solution to etch substrate which should havebeen protected further resulting in a significant loss of resolution.

We claim:
 1. A method of forming a patterned layer on a substratecomprising:(a) coating the subject with a composition comprising aterpolymer comprising an organic monomer component and sulfur dioxide ina 1:1 molar ratio, said monomer component being comprised of3-methylcyclopentene, and an omega alkynoic acid represented by theformula

    CH.tbd.C--(CH.sub.2).sub.n --COOH

wherein n is an integer from 1 to 20, and a suitable solvent therefor;(b) drying said coating to form a positive recording medium; (c)irradiating selected portions of said medium and (d) developing theirradiated portions of said medium with a suitable developer.
 2. Themethod in accordance with claim 1, wherein said composition containsfrom about 1 percent by weight to about 20 percent by weight of saidterpolymer.
 3. The method in accordance with claim 2, wherein saidcomposition contains from about 6 percent by weight to about 10 percentby weight of said terpolymer.
 4. The method in accordance with claim 1,wherein said solvent is 2-methoxyethylacetate or cyclopentanone.
 5. Amethod in accordance with claim 1, wherein said medium is irradiatedwith a source of modulated electron beam.
 6. A method in accordance withclaim 1, wherein said development exposes corresponding portions of thesubstrate.
 7. A method in accordance with claim 6 additionally includingetching the exposed substrate with a suitable etchant.
 8. A positiverecording medium comprising a substrate and a radiation sensitive filmapplied to a surface of the substrate, said film comprised of aterpolymer comprising an organic monomer component and sulfur dioxide ina 1:1 molar ratio, said monomer component being comprised of3-methylcyclopentene, and an omega alkynoic acid represented by theformula

    CH.tbd.C--(CH.sub.2).sub.n --COOH

wherein n is an integer from 1 to 20.